1. Field of the Invention
The invention concerns a coolant flow control for a clutch that can be hydraulically engaged and disengaged, and in particular, a coolant flow control system which can provide coolant flow to the clutch whenever the clutch is being engaged or disengaged and, if required, for a predetermined time interval following engagement of the clutch.
2. Description of the Related Art
Wet clutches for vehicles or power take-off shafts, such as are applied, for example, in agricultural and industrial vehicles, usually contain a hydraulically actuated piston which compresses a stack of clutch disks or plates. The clutch is engaged by friction locking, which generates heat. To avoid excessive wear, the clutch is cooled by a cooling fluid, for example, cooling oil, to transfer heat away from the clutch.
Such cooling of the clutch is primarily required during the slippage phase, when the clutch linings are heated by the slip friction, that is, when the torque transmitted by the clutch is less than the torque delivered by the engine. For example, the maximum input torque may be 65% of the rated torque of the clutch, depending upon the design of the clutch. As a first approximation, the torque transmitted by the clutch is proportional to the hydraulic piston pressure, hence slippage of the clutch will occur when the piston pressure is lower than approximately 65% of the hydraulic system pressure.
One coolant flow control system for clutches of this type is shown in EP-B1-0 143 314. In that system, the coolant flow control valve can be controlled relative to the actuation of the clutch such that cooling of the clutch disks occurs only during actuation of the clutch while engaging or disengaging, that is while the clutch is partially engaged. The control is so designed that during engagement, cooling persists after the end of the engaging process for a predetermined time interval to cool the heated clutch disks adequately. Upon disengagement of the clutch, cooling is initiated at the beginning of the disengaging process and is interrupted at the end of the disengaging process.